PROJECT SUMMARY: Scientific summary: The 5-HT2-serotonin and D2-dopamine families of receptors represent essential targets for most atypical antipsychotics as well as drugs useful in treating obesity, sleep disorders, psychosis and autistic-spectrum disorders. It is currently unknown how these drugs interact with their target receptors. Here we aim to obtain high resolution structures of 5-HT2A- and 5-HT2B- serotonin and D2- and D4-dopamine receptors with an array of small molecules. These studies are both innovative technically and conceptually by providing an unprecedented understanding of the molecular and atomic details responsible for psychoactive drug actions. Technical achievements will include the structure of LSD in complex with its key serotonin receptor targets and typical and atypical antipsychotic drugs in complex with their serotonin and dopamine receptor targets. Additionally, models which attempt to explain ligand-specific properties of biased signaling from the perspective of ligand-receptor interactions will be tested by a combination of direct structural studies, molecular modeling, docking, site-directed mutagenesis and functional studies. We anticipate that our studies will reveal key ligand-receptor interactions essential for biased signaling for serotonin and dopamine receptors. Elucidating the molecular details for such interactions provides a template for the structure-guided design of novel therapeutics.